1. Field of the Invention
The present invention relates to an assembly comprised of a plurality of switching circuits each of which controls the flow of current to an electric coil, and more particularly to a three-phase inverter for feeding current to a three-phase switched reluctance motor (SR motor) though the present invention is not limited thereto.
2. Description of the Related Art
The SR motor generally includes a rotor formed in such a manner that a polar portion protrudes outwardly from a core and a stator formed in such a manner that a polar portion protrudes inwardly from a core. The rotor is comprised of an iron core formed by merely laminating iron plates, and the stator includes an electric coil concentrically wound around each pole. The respective poles of the stator operate as electromagnets and the magnetic force of the stator attracts the respective polar portions of the rotor so that the rotor is rotated. Thus, when the current feeding state of the electric coil wound around the respective poles of the stator is sequentially changed according to the rotation position of the respective poles of the rotor, the rotor can be rotated in a desired direction. Japanese Patent Unexamined Publication No. Hei. 7-274569 discloses a feed control device for controlling rotation torque in such a manner that each electric coil is connected to a switching circuit IGBT for each phase feeding in a three-phase inverter, each switching circuit IGBT is sequentially turned on to rotate an SR motor and a feed current value of each phase is adjusted by duty control of PWN.
The three-phase inverter for the SR motor is constituted by smoothing capacitors connected to a power source in parallel and three switching circuits connected to the capacitors in parallel. Each of the switching circuits is constituted by two switching transistors IGBT and two diodes connected to each end of the electric coil. When current is supplied to the motor coil of each phase, current flows from the smoothing capacitor through a bus bar and the switching circuit.
Thus, variation of voltage occurs in the switching circuit under the influence of intrinsic inductance of the bus bar and feeding in the phase to be fed and in the other phases so that a spike voltage is generated. In order to suppress this, it has been necessary to provide a snubber circuit for absorbing the spike voltage, and the capacitance of a capacitor for absorbing the spike voltage must be increased.